The Sea Slug's Guide to Plastid Adoption

Having an intimate relationship with photosynthetic microbes is a widespread strategy adopted by numerous unicellular and multicellular organisms. Some eschew a committed relationship, and simply nab the plastids, sequestering them inside vacuoles where they continue to photosynthesize for a while. Previously we reported on a ciliate that captures the algal nuclei, as well, to support the plastids, and a flagellate that seems to in the process of converting their plastid into a well-mannered organelle. What about us metazoans? So far there is only one group known to practice this kleptoplasty—the sap-sucking sacoglossan sea slugs—and, so far, only one genus among them is known to have made this a long-term relationship.

The best-studied species here is Elysia chlorotica. These naked molluscs pass their quiet lives in salt marshes from Chesapeake Bay to Nova Scotia. Eggs laid each spring hatch into planktonic veliger larvae that home-in on the one particular species of algae that they use for food, Vaucheria litorea. The larvae attach to the algae where, within 24 hours, they metamorphose into hungry juvenile slugs that begin feeding.